Propylene impact copolymers (ICP's) are commonly used in a variety of applications where strength and impact resistance are desired such as molded and extruded automobile parts, household appliances, luggage and furniture.
Propylene homopolymers alone are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
Propylene impact copolymers are typically an intimate mixture of a continuous phase of crystalline propylene homopolymer and dispersed rubbery phase of ethylene-propylene copolymer. While these so-called impact polypropylene products can be produced by melt compounding the individual polymer components, multi-reactor technology makes it possible to directly produce these products. This is conveniently accomplished by polymerizing propylene in a first reactor and transferring the polypropylene homopolymer from the first reactor into a secondary reactor where propylene and ethylene are copolymerized in the presence of the homopolymer.
A large number of processes for preparing homopropylene, known in the art, are used as the first reactor to produce the impact copolymer. These processes include slurry loop reactors, slurry CSTR reactors and gas-phase reactors. In a loop reactor, the first reaction stage consists of one or two tubular loop reactors where bulk polymerization of homopolymers is carried out in liquid propylene. The prepolymerized catalyst, liquid propylene, hydrogen for controlling molecular weight are continuously fed into the reactor in which polymerization takes place at temperatures of 60-80° C. and pressures of up to 4 MPa, preferably 3.5 to 4 MPa. The homopolymer in liquid propylene inside the loops is continuously discharged to a separation unit. Unreacted propylene is recycled to the reaction medium. For impact copolymer production, the polymer produced in the first stage is transferred to one or two gas phase reactors where ethylene, propylene and hydrogen are added to produce impact copolymers. The granules are discharged to a flashing unit for product/monomer separation.
Polymers produced during bulk/slurry polymerizations using hydrocarbon solvents, in particular polymers of low crystallinity and/or low molecular weight, are soluble in the reaction medium. This can cause considerable problems in such bulk/slurry polymerization applications. This problem can be mitigated by operating the polymerization reactor under supercritical condition as disclosed in WO 92/12182. By nature the super critical fluid has lower polymer solving power, and nearly unlimited solubility of gaseous components. Simultaneously, the separation of the recycled reaction medium and recovered polymer is simplified, because of the energy available in the polymerization product. However, supercritical operation requires handling of high pressure equipment and is energy intensive.
Production of low crystallinity and/or low molecular weight polymers also causes in difficulty in the operation of conventional flash systems. Such flash systems are highly sensitive to high soluble polymer fractions. Any non-evaporated liquid in the separation tank risks blocking the device. This is particularly true for cyclone type of devices operated at high pressures.
Propylene polymerizations in a gas-phase process was described in the article by Ross, et al. al., “An Inproved Gas-Phase Polypropylene Process.” Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, pp. 149-154. The polymerization system consists of a fluidized bed reactor, single-stage centrifugal recycle gas compressor, recycle gas condenser, catalyst feeder and product removal system. This compressor circulates reaction gas upward through the bed, providing the agitation required for fluidization, backmixing and heat removal. No mechanical stirrers or agitators are needed in Unipol process reactors. For production of impact copolymers in the gas-phase a smaller replica of the first reaction system is operated in series. The granular resin is conveyed from the polymerization system to a bin for purging with nitrogen to remove residual hydrocarbons and then to pelletizing. One of the advantages of gas phase processes is the possibility to produce high comonomer content products. High monomer content polymer inherently has low melting or softening temperature. Under such conditions in either a fluidized or stirred gas-solid phase reactor, stickiness of the olefin polymer particles or granules becomes a problem. Ethylene copolymers using propylene, butene-1, and higher alpha comonomers are prone to stickiness problems when their crystallinity is below 30% or densities less than about 910 kg/m3. The stickiness problem becomes even more critical with copolymers of ethylene and propylene having a crystalline content less than 10%. Only limited amount of the low crystallinity copolymer can be produced in the gas-phase reactor due to the sticky nature of low crystallinity copolymer. Presence of the low crystallinity copolymer also limits the polymerization temperature to a relatively low level. Low process temperatures are generally undesired due to reduced conversion efficiency and consequent increased costs of operation. The utilization of a conventional Ziegler-Natta or vanadium based catalyst compositions in such polymerizations also leads to product that is lacking in a desired level of homogeneity and randomness as to comonomer incorporation. Improved homogeneity and random comonomer incorporation is generally desired due to improved product physical properties.
Impact copolymers are typically produced in a sequential process with a combination of at least one slurry reactor and at least one gas-phase reactor. The combined processes posses all the advantages and drawbacks associated with the slurry and gas-phase processes discussed above. While these combined sequential processes are useful, it would be desirable to have an improved process for the production of impact copolymers.
To address these issues, the present invention provides a process to produce impact copolymers using fluorinated hydrocarbons in the polymerization medium of the first and/or second reactors. Such a process may reduce the drawbacks of known two step processes for forming impact copolymers.
U.S. Pat. No. 3,470,143 discloses a process to produce a boiling-xylene soluble polymer in a slurry using certain fluorinated organic carbon compounds.
U.S. Pat. No. 5,990,251 discloses a gas phase process using a Ziegler-Natta catalyst system modified with a halogenated hydrocarbon, such as chloroform.
EP 0 459 320 A2 discloses polymerization in polar aprotic solvents, such as halogenated hydrocarbons.
U.S. Pat. No. 5,780,565 discloses dispersion polymerizations of polar monomers under super-atmospheric conditions such that the fluid is a liquid or supercritical fluid, the fluid being carbon dioxide, a hydrofluorocarbon, a perfluorocarbon or a mixture thereof.
U.S. Pat. No. 5,624,878 discloses the polymerization using “constrained geometry metal complexes” of titanium and zirconium.
U.S. Pat. No. 2,534,698, U.S. Pat. No. 2,644,809 and U.S. Pat. No. 2,548,415 disclose preparation of butyl rubber type elastomers in fluorinated solvents.
U.S. Pat. No. 6,534,613 discloses use of hydrofluorocarbons as catalyst modifiers.
U.S. Pat. No. 4,950,724 disclose the polymerization of vinyl aromatic monomers in suspension polymerization using fluorinated aliphatic organic compounds.
WO 02/34794 discloses free radical polymerizations in certain hydrofluorocarbons.
WO 02/04120 discloses a fluorous bi-phasic systems.
WO 02/059161 discloses polymerization of isobutylene using fluorinated co-initiators.
EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature.
U.S. Pat. No. 3,056,771 discloses polymerization of ethylene using TiCl4/(Et)3Al in a mixture of heptane and perfluoromethylcyclohexane, presumably at room temperature.
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